Singles
<p>
Black out some of the squares, in such a way that:
<ul><li>no number appears twice in any row or column
<li>no two black squares are adjacent
<li>the white squares form a single connected group (connections
along diagonals do not count).</ul>
<p>
Click in a square to black it out, and again to uncover it.
Right-click in a square to mark it with a circle, indicating that
you're sure it should <em>not</em> be blacked out.
